1. Field of the Invention
The present invention relates to a flexible wiring board to be connected with a circuit board by means of an anisotropic conductive film and a connection structure thereof with the circuit board.
2. Description of the Prior Art
FIG. 11 shows a fragmentary perspective view of a prior art flexible wiring board at a location where connection terminals are provided. FIG. 16 shows a plan view of a prior art flexible wiring board. Generally in a flexible wiring board mounted with an integrated circuit, as shown in FIG. 11 or 16 (the integrated circuit is not shown in FIG. 11), opposite end portions, indicated by A in FIG. 11, of an edge of a substrate or base film 2 of the flexible wiring board 1 extend beyond an array of connection terminals 3. The connection terminals 3 extend perpendicular to the edge of the substrate 2 and are arranged along the edge of the substrate.
When connecting the flexible wiring board 1 having the above-mentioned structure with a circuit board by means of an anisotropic conductive film, there occurs a problem that an unstable contact resistance results at both side portions of the array of the connection terminals 3. The contact resistance of the connection terminals 3a located in the outermost positions is relatively stable. However, the contact resistances at the connection terminals 3b of the second outermost lines and the connection terminals 3c of the third outermost lines are particularly unstable.
The reason why the contact resistance of the connection terminals 3b of the second outermost lines is unstable will now be described with reference to FIGS. 13-15 which illustrate various steps of a connection process for connecting the flexible wiring board 1 and a circuit board 6.
When the flexible wiring board 1 and the circuit board 6 are connected with each other by means of an anisotropic conductive film 5, a cushioning material 7 made of silicone rubber or the like is interposed between a pressure tool 8 and the flexible wiring board 1, as shown in FIG. 13, in order to relieve the instability at the connection portions due to variations in thickness of the substrate 2 of the flexible wiring board 1 and a substrate of the circuit board 6. Therefore, when a pressure is applied by means of the pressure tool 8, the cushioning material 7 is deformed downward to distort the end portions B of the substrate 2 as shown in FIG. 14. When the pressure tool 8 is thereafter moved upward, the end portions of the substrate 2 of the flexible wiring board 1 remain deformed adhering to the circuit board 6 at portions indicated by C in FIG. 15. As a result, the connection terminals 3b of the second outermost lines are lifted by the outermost connection terminals 3a serving as a fulcrum.
A reference numeral 9 in FIG. 15 denotes conductive particles 9 included in the anisotropic conductive film 5. The conductive particles 9 are not shown in FIGS. 13 and 14 for the sake of simplicity.
FIG. 12 shows results of a reliability test of the flexible wiring board 1 of FIG. 11 done at 65.degree. C. and 95% RH with respect of the contact resistances of the connection terminals 3b of the second outermost lines in a terminal array and the connection terminals 3 of the lines Other than the second outermost lines for the substrate 2 of the flexible wiring board 1 which outwardly protrudes by 0.15 mm to 1.0 mm from each of the outermost connection terminals 3a in the terminal array. The substrate 2 used was formed of polyimide and had a thickness of 75 .mu.m.sup.t. The connection terminals 3 each were formed of a copper foil in a weight of 1 oz, and the pitch of arrangement thereof was 0.8 min.
As obvious from the test results shown in FIG. 12, the above-mentioned lift of the connection terminals 3b of the second outermost lines is a cause of the instability of the contact resistance in an initial stage of a reliability test or after the reliability test.
In order to compensate for the instability of the contact resistance, a specified number (2 to 10) of dummy terminals 12 are provided on both sides of the array of the connection terminals 3, as shown in FIG. 16. This arrangement, however, causes the following problems.
(1) It occasionally takes place that the connection terminals 3 are required to be arranged in a limited space (length) due to a restriction in product design. In such a case, provision of dummy terminals reduces the pitch of the connection terminals 3 accordingly, and the connection terminals connected at such a reduced pitch will cause leak or other failure, which results in a degraded reliability.
(2) On the other hand, when the dummy terminals are provided without reducing the pitch of arrangement of the connection terminals 3, the area occupied by the connection terminals 3 increases, resulting in a dimensional increase of the product.
Furthermore, in the conventional flexible wiring board of FIG. 16, alignment marks 13 for aligning the connection terminals 3 with respect to counterpart terminals are provided on the substrate 2 on both sides of the array of the connection terminals 3 of the signal input side and on both sides of the other array of the connection terminals of the signal output side. This increases the length of the flexible wiring board 1 in the direction in which the connection terminals 3 are arranged, and consequently increases the size of the whole product and material costs.